Fineness, as a measure of the purity, of gold. It also indicate gold proportion in an alloy. UK spelling of both carat and karat is carat. Germany spelling is karat. 24 karats or a fineness of 1000 defines pure gold. One karat equals 41.66 fineness. 22 karat gold-alloy has 22 parts gold and 2 parts other metal, usually copper. Its fineness is 916.66, 91.666 percent pure gold. An 18 karat gold-alloy is 75 percent pure gold.